The present invention relates generally to energy harvesters, such as vibration energy harvesters, for scavenging or harvesting very low levels of energy, and more particularly to circuits and methods for protecting batteries or supercapacitors in which the harvested energy is stored. The invention also relates to circuits and methods for preventing surge current damage to inductors and/or other circuit components in DC-DC converter circuits or other power management circuitry that receives energy from the outputs of energy harvesters.
Various very low power, i.e., “nano-power”, integrated circuits that require extremely low amounts of operating current have been developed which can be powered by very small amounts of power scavenged or harvested from ambient solar, vibrational, thermal, and/or biological energy sources by means of micro-energy harvesting devices and then stored in batteries or supercapacitors. (The term “nano-power” as used herein is intended to encompass circuits and/or circuit components which draw DC current of less than roughly 1 microampere.) The amount of energy available from a harvester usually is small and unpredictable, so intermediate energy storage is often required in these applications to provide for system power needs when energy from the harvester is unavailable or insufficient. Lithium batteries or supercapacitors are commonly used for such intermediate energy storage.
Prior Art FIG. 1 shows a circuit 1 including an energy harvester 2 which produces a DC voltage Vhrv on a conductor 3 that is connected to one terminal of a large filter capacitor C0 and to the input of a conventional boost converter 7-1. Boost converter 7-1 includes an inductor L0 coupled between Vhrv and conductor 4, which is connected to one terminal of a switch S0 and to the anode of a diode D0. The other terminal of switch S0 is connected to ground. The cathode of diode D0 is connected by conductor 5 to the (+) terminal of a battery or supercapacitor 6.
Suitable power management circuitry for energy harvester 2 controls switch S0 so as to provide charging of battery/supercapacitor 6 when energy is available from harvester 2 if battery/supercapacitor 6 is not fully charged to its maximum or fully-charged voltage VBAT(max). (For a typical lithium battery, VBAT(max) is 4.5 volts.)
If battery 6 is fully charged to VBAT(max), then further charging may permanently damage it. In the unprotected system of Prior Art FIG. 1 there is nothing to prevent current from harvester 2 from overcharging battery/supercapacitor 6. However, the output voltage Vhrv generated by harvester 2 should be limited to a value below VBAT(max) to prevent damage to the battery/supercapacitor. Furthermore, such limiting of Vhrv should prevent surge currents supplied by charged-up filter capacitor C0 from damaging circuit components such as inductor L0 and/or other circuit components in the power management circuitry.
Thus, there is an unmet need for a circuit and method for protecting batteries or supercapacitors in which harvested energy is stored.
There also is an unmet need for a circuit and method for preventing surge current damage to inductors in DC-DC converter circuits coupled to an energy harvester.
There also is an unmet need for a circuit and method for both protecting batteries or supercapacitors in which harvested energy is stored and preventing surge current damage to inductors in DC-DC converter circuits coupled to an energy harvester.